Frequency Division Multiple Access (FDMA) systems and methods are widely used in wireless communication systems. FDMA refers to a wireless communication technique in which a frequency spectrum is divided into a plurality of smaller frequency cells. Each cell of the spectrum has a carrier signal that can be modulated with data. This increases the amount of data that can be communicated over the spectrum, and also provides a mechanism for allocating a bandwidth to service providers.
For example, in the upcoming evolution of 3GPP (3rd Generation Partnership Project) systems, FDMA offers a promising technology for increasing the throughput performance of 3.9G uplink (UL). In an isolated cell, the gain of FDMA over WCDMA is evident. In non-isolated cells, the gain is slightly smaller and depends mainly on the required coverage area probability. An FDMA Uplink can be realized either by using single carrier FDMA (SC-FDMA) or multicarrier OFDMA (Orthogonal FDMA, OFDMA) techniques.
The performance of the uplink of FDMA and OFDMA is sensitive to non-idealities, such as a frequency error and phase noise. Generally, the frequency error is caused by Doppler shift and frequency synchronization errors between uplink and downlink transceivers. In the worst case, the frequency error caused by Doppler effect detected by a base station receiver is two times the maximum Doppler shift.
The problem related to the frequency error is severe in the uplink direction where each terminal has its own local oscillator synchronized with a base stations' local oscillator in a downlink direction. In the synchronizing phase, each terminal sees a different Doppler shift, which is added to the frequency difference between the local oscillators of the terminal and base station. Thus, the base station sees different frequency corrections from different terminals.
The non-idealities produce adjacent channel leakage. This, in turn, causes multiple access interference, which means that different users of FDMA/OFMA system start to interfere each other at the base station receiver. The higher the power differences between the received levels of different users using the adjacent bands, the greater the problem with multiple access interference.
FIG. 1 illustrates the bandwidth usage principle in a known single carrier FDMA system (SC-FDMA). A common frequency band is used by multiple user terminals. The total bandwidth 110 is, for example, 20 MHz. Each user terminal adjusts the carrier frequency and signal bandwidth 100, 102, 104, for example, according to the data rate and signal-to-interference-noise-ratio (SINR). In the SC-FDMA, the problem of multiple access interference is solved by transmit and receive filters and guard bands 106, 108 between the users. The drawback of SC-FDMA is that rather broad guard bands and long guard times are needed, which causes a high overhead. This, in turn, will decrease the spectrum efficiency of the system. The problem is greatest with the narrowest transmission bandwidths.
FIG. 2 illustrates another known way of spectrum utilization in FDMA/OFDMA systems. Users having different modulation and coding schemes (MCS) 1×16QAM⅔ USERs, 2×QPSK½ USERs, 4×QPSK⅙ USERs have been located into the frequency domain such that users having the same MCSs are close to each other and the users having different MCS are far away in the same frequency domain 100. In the receiver side a common filter is used. The problem with this approach is that the users having high received power levels (e.g. 16 QAM, effective code rate=⅔) cause strong interference to other users having low received power levels. The interference problem is more severe if there are frequency errors in the system.
Because of the foregoing reasons it is desirable to consider improvements to radio resource control in Uplink Frequency Division Multiple Access systems.